Supramolecular Interactions in the Formation of Thermotropic Liquid Crystalline Polymers - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 119

Fig. 1 Common architectures for thermotropic liquid crystalline polymers

backbone or as a pendant group. The former are referred to as main-chain
LC polymers (MC-LCPs), while the latter are termed side-chain LC polymers
(SC-LCPs). In both cases, mesogens are linked either to each other or with the
polymer backbone through a more flexible chain, often an alkyl chain, termed
the flexible spacer . In SC-LCPs, the mesogens are attached to the polymer
backbone by one end (side-chain LCP) or laterally (side-on LCP). Similarly,
MC-LCPs can be formed by two possible arrangements of the flexible spac-
ers and mesogens: the mesogens can be linked to the flexible spacer by either
their ends, or laterally. Interestingly, Griffin has used some of the latter as
an approach to design synthetic auxetic materials (i.e., materials that ex-
pand perpendicularly to the direction of the stress applied, negative Poisson
ratio) [7].
In the area of main-chain LCPs, several approaches toward spacer in-
corporation have been adopted, including copolymerization of semiflexi-
ble polyethylene terephthalate (PET) with rod-like poly(4-hydroxy benzoic
acid) [8], polycondensation of aliphatic dicarboxylic acids with mesogenic
bisphenols [9], and especially polycondensation of hydroquinones (sub-
stituted or not) with -bis benzoic acids (such as 4,4 -dicarboxy- -

diphenoxyhexane) bearing the spacer molecule [10, 11]. Additionally, semi-
flexible polyethers with comparatively lower transition temperatures have
been prepared from dibromoalkanes and mesogenic diols using a phase
transfer catalyst approach [12]. Numerous studies of the phase behavior and
physical properties of segmented LCPs has revealed a critical role of the
spacer length, along with any mesogen pendent group, in determining the
crystallinity, the LC phases observed, and the phase transition temperatures.
Perhaps the most intriguing aspect of these studies, reviewed by Sirigu, [5]
is the remarkable “odd-even” alternation in phase transition temperature(s)

α , ω α , ω